The concentration of micro-particulate matter contained in the atmosphere is used as one of indicators of air pollution. In recent years, for example, many atmospheric measurement stations have been installed to measure the concentration of the micro-particulate matter such as PM2.5 from the relation with the effects on health. The atmospheric measurement stations use, for example, a beta (β) ray attenuation type measurement apparatus or a Tapered Element Oscillating Microbalance (TEOM) type measurement apparatus to measure the concentration of the micro-particulate matter. However, since these measurement apparatuses are expensive and large in size, they are not suitable for use by, for example, individuals.
Meanwhile, there has been proposed a simple measurement apparatus using a light scattering method. Such a measurement apparatus measures the concentration of the micro-particulate matter by measuring scattered light obtained by irradiating particles with light. Since the measurement apparatus using the light scattering method is less expensive, compacter, and easier to use than the measurement apparatuses used in the atmospheric measuring stations, it is suitable for use by, for example, individuals. For this reason, the measurement apparatus using the light scattering method is frequently used to measure the environments around users, such as the working environments.
When a measurement apparatus using the light scattering method is used for a long period of time, since the surface of an optical system such as a light source or a photodetector is contaminated, the measured values becomes inaccurate. The cause of contamination is the micro-particulate matter which is a main measurement target and adheres to the surface of the optical system such as a light source or a photodetector. However, when a filter is installed in the measurement apparatus for the purpose of preventing contamination caused by the micro-particulate matter, the measurement values of the micro-particulate matter serving as the measurement target will be affected. Therefore, it is not preferable to install a filter in the measurement apparatus. Meanwhile, when the optical system is cleaned, the measurement accuracy of the measurement apparatus is recovered, but the number of processes associated with the cleaning is increased. For example, in a case of a measurement apparatus which continuously operates in an unmanned state for a long period of time, it is not preferable to increase the number of processes associated with the cleaning. However, it is difficult to measure the concentration of the micro-particulate matter with high accuracy by suppressing the influence of contamination of the optical system without increasing the number of processes associated with the cleaning.
In addition, as methods of measuring scattered light by the light scattering method, there have been proposed various methods such as a counting detection method and an analog detection method which will be described below.
The counting detection method counts individual micro-particles by detecting light pulses generated when the micro-particulate matter passes through the field of view of a photodetector. Thus, the counting detection method has higher measurement accuracy for low concentration micro-particulate matter, but has lower measurement accuracy for high concentration micro-particulate matter as compared to the low concentration micro-particulate matter. This is because, when the number of micro-particles is large, a saturation occurs due to the counting loss of micro-particles when the high concentration micro-particulate matter is measured by the counting detection method.
Meanwhile, the analog detection method measures the light intensity of reflected light obtained by irradiating a particle group with light. For this reason, the analog detection method has higher measurement accuracy for high concentration micro-particulate matter, but has lower measurement accuracy for low concentration micro-particulate matter as compared to the high concentration micro-particulate matter. In a case of measuring the low concentration micro-particulate matter by the analog detection method, due to, for example, unsteadiness of the light intensity of light emitted from a light source, the measurement accuracy of the low concentration micro-particulate matter is relatively lower than the measurement accuracy of the high concentration micro-particulate matter.
Therefore, with a simple measurement apparatus using the light scattering method, it is difficult to continuously measure the concentration of the micro-particulate matter in a range from low concentration micro-particulate matter to high concentration micro-particulate matter with high accuracy.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2009-030988 and 2006-010353.